A/D and D/A converters are well known in the art and differ in the quality and accuracy of conversion. Various factors influence the differences in quality, most especially, the sampling frequency and the number of bits of accuracy per sample. For a general discussion of real world problems experienced by A/D and D/A converters, reference is made to "Digital Signal Processing: principles, algorithms and applications", second edition by John Proakis and Dimitris Manolakis published 1992 by the Macmillan Publishing Company at pages 412-430.
A common form of A/D conversion is to utilise over sampling and pulse code modulation in a system known generally as sigma delta modulation (SDM). SDM methods are utilised in most modern digital audio systems. Referring now to FIG. 1, there is illustrated an example SDM system 1 which normally comprises two parts being a sigma delta modulator 2 and a decimator 3. The sigma delta modulator 2 normally converts an analogue input signal 4 into a 1-bit signal 5 having a high sample rate which can be, say, 64 times the final sample rate of the overall analogue to digital converter 1. The decimator 3 converts the 1-bit digital signal 5 to a digital audio signal 6 having the desired sample rate having multi-bit output. The decimator 3 normally comprises a low pass (anti-alias) filter of 1-bit signal 5.
The D/A conversion process utilising 1-bit techniques is similar to the A/D process. With reference to FIG. 2, the D/A process can normally comprise inputting a processed digital signal 7 which is interpolated and subjected to SDM sampling 11 to form a high frequency 1-bit signal 12 which is then converted to an analogue output and low pass filtered 13 to form a final analogue output 14.
For a general introduction as to the issues involved in the construction of SDM circuits and to the complexities of implementing SDM digital to analogue converters, reference is made to the standard text, "Delta-Sigma Data Converters", edited by S. R. Norsworthy et. al., published by IEEE Press Marketing, Piscataway, N.J.
It has been suggested by persons skilled in the field that digital audio systems utilising an arrangement of FIG. 1 and subsequent digital audio processing of signal 6, can produce inferior results due to the supposed destruction of much of the high frequency information in the original audio signal 4. There is growing body of evidence that frequencies normally considered outside the audible range (eg. above 20 kHz) play an important role in our perception of transient and other audio effects.